In the aircraft or airplane industry, the manufacture of relatively small sheet metal parts (2.times.2 inches up to 4.times.10 feet) represents a majority of the total number of all parts manufactured. The methods of making such parts in practice today are basically the same as the methods used thirty or forty years ago, the only difference being that in some cases numerically-controlled (NC) machines have been substituted for manually-controlled ones.
At best, those NC machines which have been developed thus far are semi-automatic in nature, because an operator is always required for loading/unloading operations, maintenance, etc. No fully-automated system has yet to be developed which can make the wide variety of different airplane parts required for the typical commercial jet.
Airplane sheet metal parts are generally cut from flat aluminum sheets ranging in size from 4.times.6 feet to 4.times.12 feet. Many of the flat parts cut from such sheets are subsequently formed into various shapes corresponding to their intended use as an airplane part. At present, three types of primary systems are used to make airplane sheet metal parts. These include blanking presses, stack router systems, and hand-routing systems.
Blanking presses are well-known as they are very common in the sheet metal industry. Their disadvantages are also well known. Mainly, they are expensive and inefficient because they require high-quality hard tooling, and setup of such tools is time consuming relative to the short run times for the number of parts pressed during a given lot run.
The typical stack router involves first stacking a plurality of sheets, and then mounting them on a caul-plate of three-quarter inch plywood, or the like. Generally, the sheets are mounted to the plywood by screws. After mounting, the profiles around each part are machined, typically by hand-control.
One disadvantage associated with the stack router system is that the resultant parts have poor edge quality and undesirable burrs. This is primarily caused by high temperatures during the cutting process, which is unavoidable because such system creates a situation where high rates of metal removal generate heat that cannot be adequately dissipated by the inherently ineffective cooling of stacked sheets.
Another disadvantage associated with the stack router is that no machining of individual sheet surfaces can be done since several sheets are stacked one on top of another. Specifically, this prevents counter-sinking or chamfering on individual sheet surfaces. A further disadvantage is that after all parts have been profiled, the stack must thereafter be removed from the caul plate, which is a labor-intensive process.
Hand-routing is another common process utilized in making sheet metal parts. This process simply requires an operator to cut each individual part using a hand-held router that is forced against a template. It goes without saying that such process is labor intensive, and also requires unique template tools for each part, both things tending to make it a costly process.
Because of the various inefficiencies associated with the above-described systems, there has been a long-felt need to develop new systems that are either fully automated or at least have a higher degree of automation than the above systems. However, this goal has been difficult to realize, mostly because airplane sheet metal parts differ in many ways from sheet metal parts used in other industries. These differences are attributable to not only the physical characteristics of the parts manufactured, but also the total volume of parts manufactured relative to the total number of part types required.
For example, although airplane sheet metal parts are mostly aluminum, several different alloys of aluminum are actually used, each having its own unique characteristics depending on part type requirements. Of these, when considering or developing automated part-making machines, it is important to consider the condition (hard or soft) of any given alloy when it is formed into a part.
Generally, the hard or tempered condition of an aluminum alloy is known as the "T" condition, and the soft condition is known as the "O" condition. Airplane parts are sometimes machined while in the "O" condition, because in such condition their softness enables them to be later formed or shaped without cracking. After forming or shaping, such parts are then subjected to heat treatment, which gives them the necessary strength for their intended structural purpose. Unfortunately, heat treatment can create part distortions and warpage, resulting in substantial hand labor for later straightening of these parts.
An alternative soft condition for an aluminum alloy is known as the "W" condition. This is an unstable condition, except for when the alloy is kept at very low temperatures (typically minus 10 degrees F). Exposing an aluminum alloy to room temperature while in the "W" condition will cause the material to gradually cure to a hard condition. However, it takes time for this to happen, and it is possible to expose "W" aluminum sheets to temperatures as high as 50 degrees F. for several hours, for example, and the material will not have enough time to harden appreciably.
It would be preferable to form airplane parts while in the "W" condition because, after each part is formed, it can then be allowed to slowly cure at room temperature, thus obtaining a desired hard condition without significant warpage and/or distortion. In order to do this, it is necessary, as a practical matter, to have the capability of both machining and forming quickly at higher temperatures closer to room temperature, because it is difficult to conduct such operations at temperatures as low as -10 degrees F.
For this reason, prior to the development of the invention disclosed here, cutting and then forming in the "W" condition has not been feasible. It has been impossible to remove flat sheets from cold storage, and machine blanks and other stock from them using the above-described systems, and then either form the machined parts or place them back in cold storage quickly enough so that the "W" condition is maintained. That . is one of the significant advantages of the present invention. That is to say, the invention provides for extremely fast machining of flat parts in the "W" condition.
Sheet metal parts for commercial jets range in size from as small as 1.times.1 inch up to 4.times.12 feet. Ninety percent of the total volume of parts required in airplane manufacturing falls within the much-narrower range previously mentioned, i.e. 2.times.2 inches up to 10.times.20 inches. However, the labor spent on such parts is not proportionate to the 90% volume figure. Instead, perhaps 40 to 50% of the total labor spent in sheet metal part fabrication is used to produce 90% of the parts by volume. The remaining 60 to 50% of the total labor is used to produce the remaining 10% of the part volume. This is mostly caused by the time required for tooling set-up of the latter, since their size/shape is different from the majority of parts.
Therefore, in addition to a long-felt need to develop machining systems that can work with parts in the "W" condition described above, it is also important to develop automation methods that do not rely on part-type tooling, but instead are controlled by data created for each part type, thus making it easy for the same machining system to machine a variety of parts by using only standardized tools.
A machining system that works in this way makes it easy to add, amend or substitute the data for any given part type. The result is quick and easy set-up when switching from one part type to another, creating substantial time and labor savings. As will become apparent, the present invention provides such a system, and is one that has long been needed in the airplane manufacturing industry.
Before the specific details of the invention are described, it is first appropriate to mention that a machine system built in accordance with the invention is particularly well-suited to be used in conjunction with punch-nibbler and sheet deburring machines as part of an overall sheet metal fabrication facility. The underlying idea with the machine system disclosed here, and the other two just mentioned, is to process as many parts as possible while they are contained together, or nested, on a single sheet of standard size (four by six, or four by twelve feet in the examples described above).
Briefly, and by way of explanation, a punch-nibbler machine is a commercially-available machine that performs the following functions: hole punching, including round, rectangular, polygonal holes, etc.; dimpled or extruded holes, such as holes formed with rims around their periphery; louvers; threaded holes; stamping of part numbers and/or similar marks; notches or cut-outs of various shapes; and rough cuts along the periphery of a part. In some cases, a punch-nibbler can be used to make a finish cut along a part's periphery.
It is typical that a punch-nibbler be used to make rough, periphery cuts, especially for larger parts, i.e. a sequence of slightly overlapping holes either with a curved, oblong punch aligned as closely as possible to the part's profile. If the part is not designed to be used in a sonic area of the airplane, the part's edge may not need subsequent machining. However, such parts may still need to be deburred via a sheet deburring machine.
Typical deburring machines have scotch-brite (TM) type brushes that remove small burrs from part surfaces. In the case of clad aluminum, for example, where a mirror finish must be retained on the part, it is not possible to employ a deburring machine's brushes. In such situations, parts that have first been run through a punch-nibbler machine could be left in place in the larger sheet, held by bridges or tabs, and thereafter processed in the machining system disclosed here. This is a further advantage of the invention in that it is possible to use it for machining burr-free edges in certain situations.
As will become apparent, a machining station in accordance with the invention can perform various machining operations, including profile cutting, either directly or as a finish cut after nibbling; breaking of sharp profile edges; machining on sheet upper and lower surfaces, including countersinking, counterboring, chamfering, face milling, grooving, etc.; hole reaming to precision tolerance; part-making, for assisting automation in subsequent processes; and tab cut-off for releasing parts onto an output conveyor. These various aspects of the invention, which make it well-suited for use in a large-scale part-making facility, along with the other machines described above, will become apparent upon review of the disclosure set forth below.